Broadband population inversion in solid state NMR

Abstract

We present theory, simulations and experimental demonstrations of composite π pulses for population inversion in coupled spin systems such as occur in solid state NMR. The composite π pulses are phase-shifted rf pulse sequences designed to invert spins over a larger range of dipole or quadrupole couplings than a conventional π pulse, for a given rf power. We discuss a previously proposed theory for constructing composite pulses, in the specific context of solids. Two particular sequences 450180909018018090450 and 18001801201800 are examined in detail. Their performance in coupled spin systems of various sizes is evaluated in simulations. Experiments are performed on two solid compounds, Ba(ClO3)2⋅H2O and C4O4H2. The results reveal markedly less spectral distortion after composite pulse inversion than after conventional π pulse inversion at low rf powers.